A. Field of the Invention
The invention relates to optical sweeping systems and more particularly to non-impact printers and facsimile machines of the laser type.
B. Description of the Prior Art
In optical sweeping systems, a collimated light beam, for example, a laser light beam is reflected by a rotating multi-faceted mirror. The rotating mirror causes the reflected light beam to sweep periodically in a fan like fashion across a moving target surface, the end of the beam tracing out a plurality of straight lines thereon. By sweeping, it is meant that the direction of the light beam reflected from a side of the mirror changes over a fixed angle as the mirror rotates until the light beam moves off the side of the mirror onto the next side of the multifaceted mirror to begin the sweep all over again. The direction of the beam changes at a constant rate, that is, the direction of the beam changes by the same angular amount for a given time interval throughout the entire sweep. Because of this and also because the distance to the straight line path on the target surface from the reflection point on the mirror is greater at the ends of the sweep than at the middle, the end of the reflected beam covers a greater distance along a straight line path, at the ends of the sweep than at the middle during the same time interval. This is often referred to in the art as the tangential velocity of the beam and it varies during the sweep, being a higher velocity at the ends of the sweep and less in the middle.
In laser printers, the light beam is modulated before reflection, in accordance with selected patterns of bit signals which represent alphanumeric characters which are stored in a character generator memory as a matrix of ones and zeros. A character clock signal gates the individual bit signals from the character generator and the bit signals are transmitted to an R.F. signal source, which , for example, transmits R.F. signals when high bit signals (ones) are received and no R.F. signals when low bit signals (zeros) are received. Each sweep of the light beam is modulated in accordance with at least one row of ones and zeros of a plurality of matrices stored in a character generator memory for imaging as a portion of a line of alpha-numeric characters on a photosensitive surface.
The R.F. signals are transmitted to a light beam modulator which is positioned in the path of the collimated light beam and which causes a portion of the light beam to be diffracted through a specific angle (called the Bragg angle) along a deflected path when R.D. signals are present at the modulator. The portion of the beam traveling along the deflected path is called the first order beam while the undeflected beam is called the zero order beam. The zero order beam is always present although with less energy when the first order beam is present.
Together, the first and zero order beams form a modulated light beam.
The modulated light beam then passes through an optical system that controls the focus and size of the beam, and directs the beam to a multifaceted mirror where the beam is swept as described above. As the modulated light beam follows the straight line path on the photosensitive surface during the sweep, the zero order beam is prevented from impinging thereon. When it is desired to image a dot along the straight line path, the first order beam is activated in the above described manner. Otherwise, a space is left on the straight line path. If the character clock signal which gates the individual bit signals from the character generator to thereby activate the first order beam, has a constant frequency, then the separation between adjacent dots and spaces at the ends of the straight line path is greater than at the center. This is so because of the variation in tangential velocity previously described. This variation causes spreading of the subsequently imaged characters located at opposite ends of the straight line path on the photosensitive surface. That is, characters at opposite ends of the path are wider than the same characters at the center. The non-uniformity gives an undesirable appearance and result.
Various approaches have been tried to correct this problem. For example, U.S. Pat. No. 3,835,249, issued to Dattillo et al, discloses a synchronization signal for use with a scanning light beam. It includes means for splitting the main beam, an optical foci, and a light detection device. The split beam is passed through the grating and impinges on the light detection device which is located at the second foci of the optical system. The output signal from the light detection device provides clocking signals for information passing into or out of the light beam. The periodic spacing of the optical grating lines along a straight line provides information with respect to the variation in tangential velocity. A disadvantage of the Dattilo device is its requirement that the fonts used for storing alphanumeric characters be related to the optical line grating since the grating determines the clocking rate. This, of course, reduces its flexibility.
Another prior art system is disclosed in U.S. Pat. No. 4,019,186 issued to Dressen et al. It discloses a light beam motion pick-up device comprising a light transmission rod having a plurality of marks thereon. A portion of the beam is scanned along the rod and whenever it strikes one of the marks, it is scattered and the scattered light travels inside the rod to a photo-electric element which provides timing signals. However, in order to provide a clocking signal for each dot or space forming a character it is necessary to provide a mark for each such dot or space. This could amount to as many as 200 or more marks per inch and is therefore not easily achievable.
A still further prior art system is disclosed in U.S. Pat. No. 4,307,409 issued to Nelson L. Greenig et al on Dec. 22, 1981 and assigned to the assignee of the present invention. Because of this commonality of assignment, the entire content of that patent is considered to be incorporated into this specification by this reference.
U.S. Pat. No. 4,307,409 also disclosed a beam feedback synchronization system for optical sweeping mechanisms. That system includes a multifaceted rotating reflection mirror which acts on a collimated light beam such as a laser beam to cause it to sweep periodically in parallel straight lines across a rotating photoconducting drum. A portion of the light beam is split off and caused to sweep across an equispaced linear array of fiber optic apertures held in place by a fiber optic assembly. The light entering the fiber optic apertures is carried along fiber optic elements to one or more photodetectors which generate periodic electrical signals in response thereto. The frequency of occurrence of the electrical signals from the photodetector is a measure of the velocity of the sweeping light beam across the fiber optic array. A phase locked loop circuit connected to the photodetector(s) provides a character clock signal which is synchronized to the electrical signals received from the photodetector and compensates for variations in the speed of the light beam across the fiber optic array. The phase locked loop further comprises a fast synchronization circuit which provides immediate synchronization between the character clock signal and the first electrical signal occurring at the beginning of a new light beam sweep. In addition, hold circuitry is provided which holds the frequency of the character clock signal coming from the phase locked loop constant during the dead time occurring between laser light beam sweeps.
The foregoing illustrates limitations of the known prior art. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations as set forth above. Accordingly, a suitable alternative is to provide an improved beam position feedback sensor apparatus and system particularly useful with light beam type printers.